on IRC.
Did you get the sed correct?
There are only three instances of pthread_stubs_possible in
configure.ac.
The sed uses:
/pthread_stubs_possible=/
Did you miss the = sign?

I won't say no, but it is unlikely as it was copy and paste, but I'm not in
front of a machine where I can test ATM. The result was similar to this block
in configure:
....
if test "x$pthread_stubs_possible" == "xyes"; then